Field of the Invention
The subject matter herein relates to solid state lighting (SSL) fixtures and, more particularly, to SSL fixtures having integrated driver circuitry.
Description of the Related Art
There is an ongoing effort to develop systems that are more energy-efficient. A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting, a large portion of which is general illumination (e.g., downlights, flood lights, spotlights and other general residential or commercial illumination products). Accordingly, there is an ongoing need to provide lighting that is more energy-efficient.
Solid state light emitters (e.g., light emitting diodes) are receiving much attention due to their energy efficiency. It is well known that incandescent light bulbs are very energy-inefficient light sources; about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs but are still less efficient than solid state light emitters, such as light emitting diodes.
LEDs and other solid state light emitters may be energy efficient, so as to satisfy ENERGY STAR® program requirements. ENERGY STAR program requirements for LEDs are defined in “ENERGY STAR® Program Requirements for Solid State Lighting Luminaires, Eligibility Criteria-Version 1.1”, Final: Dec. 19, 2008, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
In addition, as compared to the normal lifetimes of solid state light emitters, e.g., light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000 hours. Fluorescent bulbs have longer lifetimes than incandescent lights (e.g., fluorescent bulbs typically have lifetimes of 10,000-20,000 hours), but provide less favorable color reproduction. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Where the light-producing device lifetime of the light emitter is less than the lifetime of the fixture, the need for periodic change-outs is presented. The impact of the need to replace light emitters is particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, highway tunnels) and/or where change-out costs are extremely high.
LED lighting systems can offer a long operational lifetime relative to conventional incandescent and fluorescent bulbs. LED lighting system lifetime is typically measured by an “L70 lifetime”, i.e., a number of operational hours in which the light output of the LED lighting system does not degrade by more than 30%. Typically, an L70 lifetime of at least 25,000 hours is desirable, and has become a standard design goal. As used herein, L70 lifetime is defined by Illuminating Engineering Society Standard LM-80-08, entitled “IES Approved Method for Measuring Lumen Maintenance of LED Light Sources”, Sep. 22, 2008, ISBN No. 978-0-87995-227-3, also referred to herein as “LM-80”, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein, and/or using the lifetime projections found in the ENERGY STAR Program Requirements cited above or described by the ASSIST method of lifetime prediction, as described in “ASSIST Recommends . . . LED Life For General Lighting: Definition of Life”, Volume 1, Issue 1, February 2005, the disclosure of which is hereby incorporated herein by reference as if set forth fully herein.
Heat is a major concern in obtaining a desirable operational lifetime for solid state light emitters. As is well known, an LED also generates considerable heat during the generation of light. The heat is generally measured by a “junction temperature”, i.e., the temperature of the semiconductor junction of the LED. In order to provide an acceptable lifetime, for example, an L70 of at least 25,000 hours, it is desirable to ensure that the junction temperature should not be above 85° C. In order to ensure a junction temperature that is not above 85° C., various heat sinking schemes have been developed to dissipate at least some of the heat that is generated by the LED. See, for example, Application Note: CLD-APO6.006, entitled Cree® XLamp® XR Family&4550 LED Reliability, published at cree.com/xlamp, September 2008.
Although the development of solid state light emitters (e.g., light emitting diodes) has in many ways revolutionized the lighting industry, some of the characteristics of solid state light emitters have presented challenges, some of which have not yet been fully met. For example, solid state light emitters are commonly seen in indicator lamps and the like, but are not yet in widespread use for general illumination.
Accordingly, for these and other reasons, efforts have been ongoing to develop ways by which solid state light emitters, which may or may not include luminescent material(s), can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide solid state light emitters that are improved, e.g., with respect to energy efficiency, color rendering index (CRI Ra), contrast, efficacy (1 m/W), cost, duration of service, convenience and/or availability for use in different aesthetic orientations and arrangements.
In order to encourage development and deployment of highly energy efficient solid state lighting (SSL) products to replace several of the most common lighting products currently used in the United States, including 60-Watt A19 incandescent and PAR 38 halogen incandescent lamps, the Bright Tomorrow Lighting Competition (L Prize™) has been authorized in the Energy Independence and Security Act of 2007(EISA). The L Prize is described in “Bright Tomorrow Lighting Competition(L Prize™)”, May 28, 2008, Document No. 08NT006643. The L Prize winner must conform to many product requirements including light output, wattage, color rendering index, correlated color temperature, expected lifetime, dimensions and base type.
Presently, the predominant lighting fixture in specification homes is the dome light. Because the dome light is comparatively inexpensive, provides adequate light in a relatively even distribution, and in some cases does not require anything other than a simple junction box in a ceiling to install, it is in widespread use.
Currently, dome lights typically use two 60 Watt A-lamps shining light through a low optical efficiency dome to deliver between 600-900 lumens into the space. One approach to providing an energy-efficient replacement for such a fixture would be to simply replace the A-lamps with LED lamps. Such an approach could provide a drop from 120 Watts to 24 Watts (2×12 W) or less. Utilizing LED lamps in a traditional dome light would generally result in the premature failure of those lamps, because incandescent dome lights are not constructed in a manner that would allow the LED lamps to run cool.
Thus, there is a need to develop efficient LED fixtures that are lightweight, have a low height profile, and are easy to install in existing lighting spaces, such as ceiling or wall recesses, for example.
Cree, Inc. produces a variety of recessed downlights, such as the LR-6 and CR-6, which use LEDs for illumination. SSL panels are also commonly used as backlights for small liquid crystal display (LCD) screens, such as LCD display screens used in portable electronic devices, and for larger displays, such as LCD television displays.
SSL devices are typically powered with a DC signal. However, power is conventionally delivered in DC form. It is therefore generally desirable for a solid state light fixture to include an AC-DC converter to convert AC line voltage to a DC voltage.
Boost converters can be used to generate DC voltage from an ac line voltage with high power factor and low total harmonic distortion. The voltage of an LED-based load may be higher than the peak of the input (line) ac voltage. In that case, a single-stage boost converter can be employed as the driver, achieving high power efficiency and low cost. For example, a power factor corrected (PFC) boost converter which converts 120V ac, 60 Hz, to 200-250V dc output could be used to drive an array of high-voltage (HV) LEDs at a power level of 10-15 W.
For general lighting applications, it is desirable for an SSL apparatus to be compatible with a phase-cut dimming signal. Phase-cut dimmers are commonly used to reduce input power to conventional incandescent lighting fixtures, which causes the fixtures to dim. Phase-cut dimmers only pass a portion of the input voltage waveform in each cycle. Thus, during a portion of a phase-cut ac input signal, no voltage is provided to the fixture.
Compatibility with phase cut dimming signals is also feasible for LED drivers based on boost converters. One low cost approach is to use open-loop control, which means a driver will not respond to the LED current decrease due to phase cut dimming, but rather keep the preset input current during dimmer conduction time. In this way, a “natural” dimming performance is achieved, and input power, and thus LED current, will reduce as the dimmer conduction time decreases. Another approach uses closed-loop control for the driver. As control loops are complete and in effect, these drivers will try to compensate the input power decrease due to dimmer phase cut. In order to dim LEDs in these cases, the control loops should be saturated so that the input current cannot increase. The control loop saturation can be realized by clamping the output of an error amplifier, for example.